An element in which a Peltier effect or a Seebeck effect is utilized is used as a thermoelectric conversion element. The thermoelectric conversion element has a simple structure, and the thermoelectric conversion element is easily handled to be able to maintain a stable characteristic. Therefore, the thermoelectric conversion element attracts attention of a wide variety of applications. Particularly, because the thermoelectric conversion element can perform to cool a limited part and to control a temperature of the limited part near room temperature, accurately, as an electronic cooling element, researches are widely conducted toward applications of optoelectronics and a an isothermal treatment of a semiconductor laser.
Conventionally, as illustrated in FIG. 8, in configuration of the thermoelectric conversion element used in electronic cooling and thermic generation. Plural pn junction pairs are arrayed in series, the pn junction pair being configured so that p-type thermoelectric material 804 is in contact with n-type thermoelectric material 805 through junction electrode 806. In FIG. 8, the sign 803 designates a substrate, the sign 801 designates a current introduction terminal (positive electrode), the sign 802 designates a current introduction terminal (negative electrode), and the sign H designates an arrow that indicates a heat flow direction. The thermoelectric conversion element illustrated in FIG. 8 is configured such that, depending on a direction of a current passed through a junction portion, one end portion is heated while the other end portion is cooled.
A material that has a large performance index Z in a usage temperature range is used as the thermoelectric conversion element. The performance index Z is expressed by a Seebeck coefficient “a” that is of a unique constant of a substance, a specific resistance “r”, and a thermal conductivity “K” (Z=a2/rK). Usually, a Bi2Te3 system material is used in the thermoelectric conversion element, and a crystal of the Bi2Te3 based material has a significant cleavage property. Therefore, when the thermoelectric conversion element is subjected to processes such as slicing and dicing in order to obtain the thermoelectric conversion element from an ingot, a yield may be significantly degraded due to a crack and a chip.
The following method is attempted to solve the problem. The method is for producing a thermoelectric conversion module comprising the steps of: mixing, a material powder having a desired composition; heating and melting the material powder; solidifying the melted material powder to form a solid solution ingot of a thermoelectric semiconductor material having a rhombohedral structure (hexagonal structure); crushing the solid solution ingot to form solid solution powders; homogenizing particle diameters of the solid solution powders; pressurizing and sintering the solid solution powders whose particle diameters are homogenized; and, plastically deforming and flatting the powder sintered body under a hot condition to orient a crystal of the powder sintered body toward a crystal orientation in which an excellent performance index is obtained (a step of hot upset forging) (for example, see PTL 1).
A shape of each thermoelectric material chip used as the thermoelectric conversion element is a cuboid whose one side ranges from hundreds micrometers to several millimeters. Recently, in the thermoelectric conversion element that is used under near room temperature including a temperature difference of tens degrees, it is said that the thermoelectric conversion element having the size and thickness of tens to hundreds micrometers has high performance (for example, see NPL 1).
The number of pn junction pairs in one thermoelectric conversion element is up to hundreds, and density of the pn junction pair is up to tens pairs/cm2. Increasing the number of pn junction pairs becomes a necessary factor in order to improve thermoelectric conversion performance and in order to extend applications of the thermoelectric conversion element. Particularly, in power generation in which a small temperature difference is utilized, a generated electromotive force is proportional to the number of pn junction pairs. Therefore, desirably the number of pn junction pairs that are connected in series in the thermoelectric conversion element is increased as many as possible in order to take out a high voltage from the thermoelectric conversion element.
In the case in which the thermoelectric conversion element is used as a cooling element or a temperature control element, a current passed through the thermoelectric conversion element is increased with decreasing number of series-connected thermoelectric material chips. Therefore, it is necessary to make wiring or a power supply larger. Accordingly, desirably the number of series-connected thermoelectric material chips is increased as many as possible.
FIGS. 9A to 9E illustrate a conventional method for producing the thermoelectric conversion element in which the number of thermoelectric material chips per unit area (chip density) is increased while the size of the thermoelectric material chip is reduced.
In a bump forming process (a), solder bumps 602 are formed in both surfaces of plate-like or rod-shaped p-type or n-type thermoelectric material wafer 601. In an electrode wiring process (b), electrode wiring 301 is formed in a surface of substrate 101. In connecting process (c), thermoelectric material wafer 601 in which solder bumps 602 are formed through the bump forming process (a) is disposed in the face of substrate 101. Then electrode wiring 301 on substrate 101 and thermoelectric material wafer 601 are connected by soldering. FIG. 9C illustrates the connecting of p-type or n-type thermoelectric material wafer 601 and electrode wiring 301 on substrate 101. For example, when FIG. 9C illustrates the connecting of p-type thermoelectric material wafer 601 and electrode wiring 301 on substrate 101, similarly n-type thermoelectric material wafer 601 and electrode wiring 301 on substrate 101 are also connected.
In a cutting and removing process (d), connected thermoelectric material wafer 601 is cut and removed as needed basis such that electrode wirings 301, to which different types of thermoelectric material chips should be connected, emerge. Through the cutting and removing process (d), substrate 101 is prepared. On the substrate 101, p-type thermoelectric material chip 603 is connected to predetermined electrode wiring 301, and electrode wiring 301, to which n-type thermoelectric material chip 603 should be connected, emerges on the surface of substrate 101. Similarly, substrate 101 is prepared. On substrate 101, n-type thermoelectric material chip 603 is connected to predetermined electrode wiring 301n, and an electrode, to which a p-type thermoelectric material chip should be connected, emerges on the surface of substrate 101.
In an assembling process (e), for two substrates 101, surfaces, to each of which thermoelectric material chip 603 is connected, face each other. Thermoelectric material chips 603 are aligned to predetermined positions where thermoelectric material chips 603 should be connected with electrode wirings 301. A tip end of thermoelectric material chip 603 of one of substrates 101 is connected to electrode wiring 301, which corresponds to the chip, on the other substrate 101. Therefore, the thermoelectric conversion element including the pn junction pair in which the metallic electrode is interposed therebetween is formed (see PTL 2).
However, because a wafer is cut and removed to prepare the thermoelectric material chip whose section is small in a surface which is parallel to the substrate of the thermoelectric material chip, the conventional configuration has a problem in that the chip is broken during the cutting and removing process and during use. Additionally, the thermoelectric material chip is prepared by cutting and removing the wafer, which results in another problem in that the yield of the thermoelectric material is degraded.
In addition to the above thermoelectric conversion element, there is well known a thermoelectric conversion element in which p-type thermoelectric conversion material layers and n-type thermoelectric conversion material layers are alternately stacked with an insulating layer such as the substrate interposed therebetween. A thermoelectric conversion element, in which the p-type thermoelectric conversion material layers and the n-type thermoelectric conversion material layers are electrically connected in series at ends of the layers, is well known as the stacked type thermoelectric conversion element (for example, see PTLs 3 to 9). A thermoelectric conversion element, in which the p-type thermoelectric conversion material layers and the n-type thermoelectric conversion material layers are electrically connected in series at end portions of the layers in a direct manner or by surface contact in which a conductive layer is interposed, is also well known as the stacked type thermoelectric conversion element (for example, see PTLs 10 to 12). A method for forming the Bi2Te3 based material on the insulating substrate such as polyimide by sputtering is well known as a method for forming the layer of the thermoelectric conversion material (for example, see PTLs 13 and 14 and NPL 2).